The present disclosure relates generally to wearable devices and methods for measuring a photoplethysmographic signal. Most soft tissue will transmit and reflect both visible and near-infrared radiation. Thus, if light is projected onto an area of skin and the reflected light detected after its interaction with the skin, blood, and other tissues, time varying changes in light absorbance can be observed. This time varying light absorbance signal (photoplethysmographic or “PPG” signal) may be affected by a number of factors, some of which include the optical properties of the tissues and blood at the measurement site, and the wavelength of the light source.
PPG signals may be used in applications such as heart rate and oxygen saturation determination, and are highly susceptible to motion noise caused by bulk body movements and internal tissue motion. Although forces created by bulk body movements can be detected by an accelerometer, flexion, for example, of the finger or hand can generate internal tissue forces within the wrist that, for a wrist-worn device, would not necessarily be detected by an accelerometer (e.g., flexion can occur while the limbs are stationary).
PPG signals may be also be affected by non-pulsatile signal artifacts. For a typical PPG detector (photodetector), the detector is sensitive to light arriving from all angles. However, some of the arrival angles include light containing more pulsatile signal than other angles. Perfusion index (PI) is a numerical assessment of pulse strength (pulse amplitude) at a monitoring site, and is generally the ratio of received modulated light to received unmodulated light at a photodetector or sensor. In a clinical setting, PI may be used to quickly evaluate the appropriateness of a sensor application site, where the site with the highest PI number is used. Placing the sensor at the site with a high PI may result in a higher quality PPG signal that is not contaminated with artifacts due to internal tissue motion or a non-pulsatile signal. However, it may not always be feasible to move or reposition a wearable device in order to find a favorable monitoring site. Accordingly, having new wearable devices and methods for measuring a PPG signal would be beneficial.